This invention relates to petroleum wells, and more particularly, to completing a well to a petroleum bearing formation lying deep within the earth.
The Government has rights in this invention pursuant to Contract No. W-7405-ENG 36 (S-53, 311 and W(I)-112-79) awarded by the U.S. Department of Energy.
The ever increasing demand for energy has created an equivalent demand for deep drilling. The search for liquid or gaseous fuels at depths of 30,000 feet is now possible, but frequently drilling must be terminated at depths far less than this, not just because of equipment failures, but because of geological conditions that frustrate current technology.
The technological limitations to deep drilling include the following: (1) the loss of circulation, i.e. drilling fluid loss into the formation; (2) geopressurized zones that cause the blowout of the drilling fluid and drill string and consequent destruction of the rig; (3) excess weight of the drill string; and (4) the necessary sequential stepping down in size of the casing.
The most common failure in drilling is the loss of circulation due to the drilling fluid breaking into the surrounding formation. A drilling mud of high density is desirable because it can cap or contain any gas deposits with a static head pressure corresponding to its density and column height. On the other hand, many zones are far weaker than the mud pressure and will fracture with consequential loss of the drilling mud into the formation. In addition when drilling, the drilling fluid must circulate at relatively high velocity in order to carry away the cuttings and, at the same time, cool and lubricate the drilling bit. At great depths, where the temperature becomes high, the cooling requirement becomes paramount, and the required velocity of the fluid is large. The two conditions of a very long return path length of the mud between the drill stem and the casing wall and the high velocity of the mud mean that there will be a large pressure drop due to fluid flow friction. This circulation pressure drop, of course, also occurs inside the drill string on the way down to the drill bit, but the strength of the drill pipe contains this fraction of the circulation pressure drop. The remaining fraction of the circulation pressure drop must add to the hydrostatic head of the mud, and therefore increases the likelihood of wall fracture and fluid loss. Very often the balance between these pressures is highly critical, and the rate of drilling may be severely constrained or even terminated because of the inability to find a satisfactory mud density that allows for containment, circulation, and lack of fluid loss.
Geopressurized zones are important in the development of petroleum reserves because there is good reason to believe the bulk of future hydrocarbon reserves may be located in such formations. A geopressurized zone is one where the pressure of the contained fluid or gas is roughly equal to the overburden pressure. Overburden pressure corresponds to the weight of the overlying strata or rocks. It generally corresponds to 1 psi per foot of depth at a mean density of 2.0. Most rock at intermediate depths is porous, and the pores are filled with water. The hydrostatic head of this water is one half that of the overburden rock, and sometimes it is significantly less than this because the water table may not reach to the surface. Any trapped hydrocarbons will in general be at a pressure corresponding to the hydrostatic head of the water and hence, one half that of the overburden pressure. The rock strength surrounding the pores supports the difference in pressure between overburden pressure and pore pressure. When this difference in pressure becomes greater than the particular rock strength, a pore fluid of different pressure than the overburden cannot easily exist, and the tendency is for the rock at greater depth to become dense without a separate pore phase. If a gas or fluid is trapped at this depth, then it is likely to come into pressure equilibrium with the overburden pressure of the rock and form a geopressurized zone. Since this pressure is likely to be considerably greater than the hydrostatic drilling fluid pressure, the pressure may force the drilling apparatus rapidly up the well bore, causing a "blowout".